This invention relates to a sintered silicon carbide/carbon composite ceramic body and a process for making such a body. It particularly relates to pressureless-sintered, substantially homogeneous silicon carbide/carbon composite ceramic bodies, and more particularly, to those which have a very fine grain polycrystalline microstructure, and to a process for pressureless sintering of a composite ceramic body having a fine grain microstructure which process is relatively insensitive to sintering temperature and time in regard to the effect of these on the grain size or growth of grain. The microstructure of composites according to the invention is relatively unaffected upon subsequent exposure to the temperatures near to sintering temperature as compared to that of known pressureless sintered silicon carbide/carbon bodies.
The chemical and physical properties of silicon carbide make it an excellent material for high temperature structural applications. These desirable properties include good oxidation resistance and corrosion resistance, high heat transfer coefficient compared to metals and other ceramics, low expansion coefficient compared to metals, high resistance to thermal shock and high strength at elevated temperatures. Undesirable characteristics of known bodies of essentially pure silicon carbide, particularly those produced by known pressureless sintering processes, include inability to be electrical discharge machined at an acceptable rate, poor electrical conductivity, high sensitivity of microstructure to sintering conditions, growth of microstructure upon extended or repeated exposure to high temperatures, and a need to precisely estimate the amount of shrinkage which will occur upon sintering to avoid retooling if different dimensions than estimated result or are desired.
It is, therefore, believed desirable to enable the production of silicon carbide/carbon composite ceramic bodies having a density which is a high percentage of the theoretical density and suitable for engineering material uses, such as for example, high temperature applications involving relatively moving parts made to close dimensional tolerances. Silicon carbide has been identified as a preferred material for such applications (for example, refer to U.S. Pat. No. 4,312,954). Silicon carbide/carbon composite ceramic bodies according to the present invention may, in some instances, be even more advantageously employed than substantially pure silicon carbide bodies because these composite bodies in certain embodiments have a very fine grain microstructure that is easy to obtain on a repetitive basis because the shaped green body and process used in the manufacture of such composite bodies is relatively undemanding of exact temperature/time control during sintering. It is believed that certain embodiments will exhibit greater resistance to thermal shock and can withstand greater temperature differentials than known sintered silicon carbides. Some embodiments provide a self-lubricating characteristic which may be advantageously employed, for example, in mechanical seals. Other factors being equal, an ultrafine polycrystalline grain structure is desirable because it increases strength and resistance to mechanical impact loading, which properties depend upon the largest flaw present in a particular sintered ceramic body of a given material. The electrical conductivity of certain embodiments of the invention enables electrical discharge machining of these bodies as well as other electrical applications, e.g. high temperature heating elements for an inert atmosphere. This represents a significant advance in that sintered silicon carbide bodies previously were machined using expensive diamond tools which caused damage to the surface of the body being machined resulting in lower rupture resistance. The fracture toughness of certain embodiments exceeds that of known pressureless-sintered silicon carbide having a density that is a similar degree of theoretical density. Use of the process of the present invention provides lower shrinkage and/or a method to control shrinkage and porosity of sintered silicon carbide composites
Composite bodies of silicon carbide/graphite have heretofore been produced by reaction bonding (also known as reaction sintering) and hot pressing. Reaction sintering involves use of silicon impregnants to upgrade the density of silicon carbide through reaction with excess carbon in the substrate. Reaction sintering is useful for many applications but is undesirable where excess silicon exuding from the silicon carbide body would be detrimental (e.g. high temperatures in excess of 1400.degree. C.). Hot pressing (the production of high density silicon carbide/graphite composite ceramic bodies by simultaneous application of heat and pressure) is impractical for complex shapes because of complex mold design and high uniaxial pressure. The pressure required (typically of the order of greater than 1000 psig) deforms the body. Difficulty or impossibility may be encountered in removing the hot pressed part from its complex mold. As the mold configuration is increased in complexity it becomes more difficult or impossible to remove the hot pressed part.
Methods for producing composite bodies of silicon carbide/graphite are disclosed in many U.S. patents. U.S. Pat. No. 2,527,829 to Leitten et al discloses a method in which coarse particulate silicon carbide is mixed with flaked graphite and a binder which melts in the temperature range of 2000.degree.-2300.degree. F. This mixture is compacted into a briquette, held together by the binder. U.S. Pat. No. 2,907,972 to Schildhauer et al describes the production of a silicon carbide/silicon refractory by reaction sintering of silicon carbide/carbon with silicon. U.S. Pat. No. 4,019,913 to Weaver et al describes siliconizing of a silicon carbide/graphite mixture at a temperature greater than 2000.degree. C. to convert the graphite into silicon carbide and results in a single phase silicon carbide body. U.S. Pat. No. 4,154,787 to W. G. Brown describes the production of a siliconized silicon carbide/carbon body particularly useful for seal rings containing free silicon which is produced by reaction bonding of a silicon carbide/carbon mixture by infiltration of silicon. U.S. Pat. Nos. 4,312,954; 4,124,667; 4,346,049; 4,179,299; 4,135,938; 4,172,109; 4,123,286; 4,135,937; 4,144,207; 4,207,226; 4,237,085 disclose silicon carbide compositions that may contain, in some instances, up to 5 percent carbon in the final sintered silicon carbide product and, in other instances, up to 6 percent uncombined carbon in the final sintered product. A body formed according to U.S. Pat. Nos. 4,135,937 and 4,135,938 may contain up to 15 percent additional carbon (beyond that in the original particulate silicon carbide) derived from graphite or carbonized organic composition. In U.S. Pat. No. 4,135,938 it is stated that it is believed that most of the additional carbon is chemically combined with the silicon carbide and additive compound (for example, BP, BN, or AlB.sub.2).
U.S. Pat. No. 3,205,043 to Taylor discloses the manufacture of dense silicon carbide articles by forming a bonded or recrystallized porous body structure of granular silicon carbide of the desired shape whose pores are impregnated in one or more cycles with carbonizable material which is thereafter carbonized followed by heating of the carbon-impregnated body in a silicon-supplying environment to cause penetration of and reaction of the silicon with the carbon within the pores of the body to form additional carbon. This body is not, however, pressureless sintered and is commonly referred to as reaction-bonded silicon carbide.
Thus, none of these patents disclose a process for the production of a fine-grained pressureless-sintered silicon carbide/carbon composite ceramic body which process includes forming a preshaped microporous ceramic body of very fine particle size silicon carbide and thereafter infiltrating the green body with a carbon source resin. None of these patents disclose a pressureless-sintered silicon carbide/carbon composite having high electrical conductivity.
Thus, while U.S. Pat. Nos. 4,312,954 and 4,179,299 and 4,346,049 disclose a fine grained pressureless-sintered silicon carbide/carbon composite ceramic body, there remains a need/desire for a process that will reliably provide a pressureless-sintered silicon carbide/carbon composite body of even finer grain microstructure than heretofore readily attainable.
U.S. Pat. Nos. 4,179,299 and 4,346,049 teach the inherent advantages of and disclose a sintered alpha, non-cubic crystalline silicon carbide ceramic body having a predominately equiaxed microstructure; in other words, more than 50 percent of the microstructure is such that the ratio of the maximum dimension of the grains of the crystal microstructure to the minimum dimension of the grains of the crystal microstructure is less than 3 to 1. These patents may also be referred to for their teaching as to the effect on crystal size of sintering temperature and time in substantially pure silicon carbide bodies containing about 2 percent by weight of carbon. These references show that it is difficult to achieve the desired fine grain size, equiaxed microstructure unless close control of the process is maintained, particularly as regards the sintering temperature This same problem and goal in the manufacture of dense, shaped articles of alpha silicon carbide is addressed in U.S. Pat. No. 4,230,497 to Schwetz et al, who disclose use of an aluminum sintering aid to reduce the need to maintain an exact sintering temperature.
U.S. Pat. No. 3,165,864 to Schulze describes a hot-pressed silicon carbide/graphite composite body having an exposed surface of high modulus ceramic and an interior of low modulus formed substantially of graphite. The composition gradually changes from an outer layer of siliconized silicon carbide to a substantially pure graphite inner layer.
U.S. Pat. No. 4,108,675 to Tomita et al describes a refractory brick made by forming and subjecting to reducing-burning a composition comprising an acid or neutral refractory material (e.g. high Al.sub.2 O.sub.3 or high ZrO.sub.2 material) as a base material, 3-10 percent of graphite powder and a binder which may be a mixture of tar and pitch or a phenolic resin. The brick can contain 2-8 percent of finely crushed silicon carbide. The pores of the brick may be filled with carbon by impregnating the burnt composition with tar or pitch and thereafter reburning.
It is, therefore, an object of this invention to provide a sintered silicon carbide/carbon composite ceramic body having a continuous phase of sintered silicon carbide and an uncombined carbon phase of amorphous or crystalline type or mixtures thereof substantially homogeneously dispersed throughout the silicon carbide matrix. Some or most of the carbon may be in the form of graphite in the sintered composite body. "Uncombined" as used herein means not chemically combined, for example, as with silicon to form silicon carbide.
It is a further object of this invention to provide such a composite body from starting materials which may include alpha phase non-cubic crystalline silicon carbide, amorphous silicon carbide or beta silicon carbide. It is well known that the alpha phase silicon carbide is more thermodynamically stable than other forms and at this time is lower in cost. Alpha-phase non-cubic crystalline silicon carbide is also much more readily obtainable than either amorphous or beta cubic silicon carbide.
It is also an object of this invention to provide a process including pressureless sintering for the production of such sintered silicon carbide/carbon composite ceramic bodies having a fine grained polycrystalline microstructure and relatively high electrical conductivity when compared to previously publicly known pressureless-sintered silicon carbide bodies.
It is a further object of this invention to enable production of sintered silicon carbide/carbon composite bodies of different dimensions from the same mold by controlled infiltration of a carbon source organic material. The amount of shrinkage induced upon sintering is reduced by the presence of an increased amount of infiltrated carbon which is added prior to sintering. A body of greater dimension may be produced by infiltrating a greater amount of such an organic material.
In this abstract, specification and claims, unless otherwise indicated, all quantities, proportions and ratios are stated on a weight basis.
The term "microporous" and related forms, as used in this specification and appended abstract and claims, refers to a characteristic of the shaped object prior to completion of sintering and means that such shaped object has interconnected microporosity which enables infiltration by a fluid such as an organic resin.